Add the lentils, carrots, celery, onion, garlic, diced tomatoes, vegetable broth, cumin, paprika, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the lentils are tender.
Stir in the spinach during the last 15 minutes of cooking so it wilts into the stew.
While the spinach wilts, bring 1 1/4 cups water to a boil in a small saucepan. Stir in the couscous, cover, and remove from heat. Let it sit for 5 minutes, then fluff with a fork.
Spoon the couscous into bowls and top with the lentil stew. Sprinkle with fresh parsley before serving.